The Wassa Gold Mine is located in the southern portion of the Ashanti Greenstone Gold Belt in Ghana, northwest of Tarkwa. The mine has been in production as an open pit since 1998, but the surface minable reserves had become exhausted and the mine started a transition to an underground operation in 2015. SRK’s ventilation team became involved with the operation following the development of the initial twin access declines. Several site visits were conducted to audit the ventilation system in order to both identify possible system improvements and to increase efficiency.
A series of time-phased network ventilation models were developed to establish the maximum airflow regime through the mine. This was used to look at the transitions around the time when new infrastructure would be developed, and to assist in matching the mine production plan to the installed ventilation system. Throughout the modeling program the production rate for the mine was increased. The models were required to identify methods by which the system could be expanded from the initial 167.6 m3/s required for initial development, to 235.1 m3/s required for the original 2,500 tpd, and then 541.0 m3/s required for the full 4,000 tpd.
The development and planning of a ventilation system is an iterative process required by refinements in the mine plan and the eventual increases in the design production rate. As the mine plan, equipment load, and production rate changes the design of the ventilation system needs to be able to be revised. This is where flexibility and adaptability become important. Designing the ventilation system based on a simplistic equipment list with utilization factors provides for a reasonable first step for project evaluation, however, when the ventilation system is being designed, the actual equipment load in operation should be used as the basis of design.
One of the benefits of establishing the ventilation model in a time phased approach is the ability to see possible future complications with the ventilation system. This is especially true during the transition from development to production. For the Wassa ventilation system an interim step in increasing the airflow through this transition period to maximize the airflow, the existing temporary exhausting four parallel fan installation was examined to re-pitch the fan blades. To accommodate this setting, the fan motors were upgraded from 90 kW to 132 kW. However, there may be a potential problem with starting four fans in parallel at a higher-pressure operating point. As fans in parallel start in sequence, it is often the third or fourth fan that cannot develop the higher pressure during startup without transitioning through the stall zone on the performance curve. As a mine develops and the required operating point moves up the fan curve, the more difficult it will be to start the parallel fans.
Once a mine enters the production phase or at least during the late stages of development the ventilation system needs to be re-evaluated to ensure that the design assumptions used in the initial modeling are correct (friction factors and leakage resistances) and that the system is operating close to how it was designed to ensure that the future fan selections are correct. As a mine plan evolves with an increased knowledge base gained through the development process the ventilation system is likely to be modified because of tweaks to the mine plan, layout, or equipment loads. A certain amount of those tweaks can be overcome by designing flexibility into the ventilation system from inception, but some changes will require the redevelopment or expansion of the ventilation system.
